Network Working Group T. Ylonen
Request for Comments: 4252 SSH Communications Security Corp
Category: Standards Track C. Lonvick, Ed.
Cisco Systems, Inc.
January 2006
The Secure Shell (SSH) Authentication Protocol
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
The Secure Shell Protocol (SSH) is a protocol for secure remote login
and other secure network services over an insecure network. This
document describes the SSH authentication protocol framework and
public key, password, and host-based client authentication methods.
Additional authentication methods are described in separate
documents. The SSH authentication protocol runs on top of the SSH
transport layer protocol and provides a single authenticated tunnel
for the SSH connection protocol.
Ylonen & Lonvick Standards Track [Page 1]

RFC 4252 SSH Authentication Protocol January 20062. Contributors
The major original contributors of this set of documents have been:
Tatu Ylonen, Tero Kivinen, Timo J. Rinne, Sami Lehtinen (all of SSH
Communications Security Corp), and Markku-Juhani O. Saarinen
(University of Jyvaskyla). Darren Moffat was the original editor of
this set of documents and also made very substantial contributions.
Many people contributed to the development of this document over the
years. People who should be acknowledged include Mats Andersson, Ben
Harris, Bill Sommerfeld, Brent McClure, Niels Moller, Damien Miller,
Derek Fawcus, Frank Cusack, Heikki Nousiainen, Jakob Schlyter, Jeff
Van Dyke, Jeffrey Altman, Jeffrey Hutzelman, Jon Bright, Joseph
Galbraith, Ken Hornstein, Markus Friedl, Martin Forssen, Nicolas
Williams, Niels Provos, Perry Metzger, Peter Gutmann, Simon
Josefsson, Simon Tatham, Wei Dai, Denis Bider, der Mouse, and
Tadayoshi Kohno. Listing their names here does not mean that they
endorse this document, but that they have contributed to it.
3. Conventions Used in This Document
All documents related to the SSH protocols shall use the keywords
"MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" to describe
requirements. These keywords are to be interpreted as described in
[RFC2119].
The keywords "PRIVATE USE", "HIERARCHICAL ALLOCATION", "FIRST COME
FIRST SERVED", "EXPERT REVIEW", "SPECIFICATION REQUIRED", "IESG
APPROVAL", "IETF CONSENSUS", and "STANDARDS ACTION" that appear in
this document when used to describe namespace allocation are to be
interpreted as described in [RFC2434].
Protocol fields and possible values to fill them are defined in this
set of documents. Protocol fields will be defined in the message
definitions. As an example, SSH_MSG_CHANNEL_DATA is defined as
follows.
byte SSH_MSG_CHANNEL_DATA
uint32 recipient channel
string data
Throughout these documents, when the fields are referenced, they will
appear within single quotes. When values to fill those fields are
referenced, they will appear within double quotes. Using the above
example, possible values for 'data' are "foo" and "bar".
Ylonen & Lonvick Standards Track [Page 3]

RFC 4252 SSH Authentication Protocol January 20064. The Authentication Protocol Framework
The server drives the authentication by telling the client which
authentication methods can be used to continue the exchange at any
given time. The client has the freedom to try the methods listed by
the server in any order. This gives the server complete control over
the authentication process if desired, but also gives enough
flexibility for the client to use the methods it supports or that are
most convenient for the user, when multiple methods are offered by
the server.
Authentication methods are identified by their name, as defined in
[SSH-ARCH]. The "none" method is reserved, and MUST NOT be listed as
supported. However, it MAY be sent by the client. The server MUST
always reject this request, unless the client is to be granted access
without any authentication, in which case, the server MUST accept
this request. The main purpose of sending this request is to get the
list of supported methods from the server.
The server SHOULD have a timeout for authentication and disconnect if
the authentication has not been accepted within the timeout period.
The RECOMMENDED timeout period is 10 minutes. Additionally, the
implementation SHOULD limit the number of failed authentication
attempts a client may perform in a single session (the RECOMMENDED
limit is 20 attempts). If the threshold is exceeded, the server
SHOULD disconnect.
Additional thoughts about authentication timeouts and retries may be
found in [ssh-1.2.30].
5. Authentication Requests
All authentication requests MUST use the following message format.
Only the first few fields are defined; the remaining fields depend on
the authentication method.
byte SSH_MSG_USERAUTH_REQUEST
string user name in ISO-10646 UTF-8 encoding [RFC3629]
string service name in US-ASCII
string method name in US-ASCII
.... method specific fields
The 'user name' and 'service name' are repeated in every new
authentication attempt, and MAY change. The server implementation
MUST carefully check them in every message, and MUST flush any
accumulated authentication states if they change. If it is unable to
Ylonen & Lonvick Standards Track [Page 4]

RFC 4252 SSH Authentication Protocol January 2006
flush an authentication state, it MUST disconnect if the 'user name'
or 'service name' changes.
The 'service name' specifies the service to start after
authentication. There may be several different authenticated
services provided. If the requested service is not available, the
server MAY disconnect immediately or at any later time. Sending a
proper disconnect message is RECOMMENDED. In any case, if the
service does not exist, authentication MUST NOT be accepted.
If the requested 'user name' does not exist, the server MAY
disconnect, or MAY send a bogus list of acceptable authentication
'method name' values, but never accept any. This makes it possible
for the server to avoid disclosing information on which accounts
exist. In any case, if the 'user name' does not exist, the
authentication request MUST NOT be accepted.
While there is usually little point for clients to send requests that
the server does not list as acceptable, sending such requests is not
an error, and the server SHOULD simply reject requests that it does
not recognize.
An authentication request MAY result in a further exchange of
messages. All such messages depend on the authentication 'method
name' used, and the client MAY at any time continue with a new
SSH_MSG_USERAUTH_REQUEST message, in which case the server MUST
abandon the previous authentication attempt and continue with the new
one.
The following 'method name' values are defined.
"publickey" REQUIRED
"password" OPTIONAL
"hostbased" OPTIONAL
"none" NOT RECOMMENDED
Additional 'method name' values may be defined as specified in
[SSH-ARCH] and [SSH-NUMBERS].
5.1. Responses to Authentication Requests
If the server rejects the authentication request, it MUST respond
with the following:
byte SSH_MSG_USERAUTH_FAILURE
name-list authentications that can continue
boolean partial success
Ylonen & Lonvick Standards Track [Page 5]

RFC 4252 SSH Authentication Protocol January 2006
The 'authentications that can continue' is a comma-separated name-
list of authentication 'method name' values that may productively
continue the authentication dialog.
It is RECOMMENDED that servers only include those 'method name'
values in the name-list that are actually useful. However, it is not
illegal to include 'method name' values that cannot be used to
authenticate the user.
Already successfully completed authentications SHOULD NOT be included
in the name-list, unless they should be performed again for some
reason.
The value of 'partial success' MUST be TRUE if the authentication
request to which this is a response was successful. It MUST be FALSE
if the request was not successfully processed.
When the server accepts authentication, it MUST respond with the
following:
byte SSH_MSG_USERAUTH_SUCCESS
Note that this is not sent after each step in a multi-method
authentication sequence, but only when the authentication is
complete.
The client MAY send several authentication requests without waiting
for responses from previous requests. The server MUST process each
request completely and acknowledge any failed requests with a
SSH_MSG_USERAUTH_FAILURE message before processing the next request.
A request that requires further messages to be exchanged will be
aborted by a subsequent request. A client MUST NOT send a subsequent
request if it has not received a response from the server for a
previous request. A SSH_MSG_USERAUTH_FAILURE message MUST NOT be
sent for an aborted method.
SSH_MSG_USERAUTH_SUCCESS MUST be sent only once. When
SSH_MSG_USERAUTH_SUCCESS has been sent, any further authentication
requests received after that SHOULD be silently ignored.
Any non-authentication messages sent by the client after the request
that resulted in SSH_MSG_USERAUTH_SUCCESS being sent MUST be passed
to the service being run on top of this protocol. Such messages can
be identified by their message numbers (see Section 6).
Ylonen & Lonvick Standards Track [Page 6]

RFC 4252 SSH Authentication Protocol January 20065.2. The "none" Authentication Request
A client may request a list of authentication 'method name' values
that may continue by using the "none" authentication 'method name'.
If no authentication is needed for the user, the server MUST return
SSH_MSG_USERAUTH_SUCCESS. Otherwise, the server MUST return
SSH_MSG_USERAUTH_FAILURE and MAY return with it a list of methods
that may continue in its 'authentications that can continue' value.
This 'method name' MUST NOT be listed as supported by the server.
5.3. Completion of User Authentication
Authentication is complete when the server has responded with
SSH_MSG_USERAUTH_SUCCESS. All authentication related messages
received after sending this message SHOULD be silently ignored.
After sending SSH_MSG_USERAUTH_SUCCESS, the server starts the
requested service.
5.4. Banner Message
In some jurisdictions, sending a warning message before
authentication may be relevant for getting legal protection. Many
UNIX machines, for example, normally display text from /etc/issue,
use TCP wrappers, or similar software to display a banner before
issuing a login prompt.
The SSH server may send an SSH_MSG_USERAUTH_BANNER message at any
time after this authentication protocol starts and before
authentication is successful. This message contains text to be
displayed to the client user before authentication is attempted. The
format is as follows:
byte SSH_MSG_USERAUTH_BANNER
string message in ISO-10646 UTF-8 encoding [RFC3629]
string language tag [RFC3066]
By default, the client SHOULD display the 'message' on the screen.
However, since the 'message' is likely to be sent for every login
attempt, and since some client software will need to open a separate
window for this warning, the client software may allow the user to
explicitly disable the display of banners from the server. The
'message' may consist of multiple lines, with line breaks indicated
by CRLF pairs.
Ylonen & Lonvick Standards Track [Page 7]

RFC 4252 SSH Authentication Protocol January 2006
If the 'message' string is displayed, control character filtering,
discussed in [SSH-ARCH], SHOULD be used to avoid attacks by sending
terminal control characters.
6. Authentication Protocol Message Numbers
All message numbers used by this authentication protocol are in the
range from 50 to 79, which is part of the range reserved for
protocols running on top of the SSH transport layer protocol.
Message numbers of 80 and higher are reserved for protocols running
after this authentication protocol, so receiving one of them before
authentication is complete is an error, to which the server MUST
respond by disconnecting, preferably with a proper disconnect message
sent to ease troubleshooting.
After successful authentication, such messages are passed to the
higher-level service.
These are the general authentication message codes:
SSH_MSG_USERAUTH_REQUEST 50
SSH_MSG_USERAUTH_FAILURE 51
SSH_MSG_USERAUTH_SUCCESS 52
SSH_MSG_USERAUTH_BANNER 53
In addition to the above, there is a range of message numbers (60 to
79) reserved for method-specific messages. These messages are only
sent by the server (client sends only SSH_MSG_USERAUTH_REQUEST
messages). Different authentication methods reuse the same message
numbers.
7. Public Key Authentication Method: "publickey"
The only REQUIRED authentication 'method name' is "publickey"
authentication. All implementations MUST support this method;
however, not all users need to have public keys, and most local
policies are not likely to require public key authentication for all
users in the near future.
With this method, the possession of a private key serves as
authentication. This method works by sending a signature created
with a private key of the user. The server MUST check that the key
is a valid authenticator for the user, and MUST check that the
signature is valid. If both hold, the authentication request MUST be
accepted; otherwise, it MUST be rejected. Note that the server MAY
require additional authentications after successful authentication.
Ylonen & Lonvick Standards Track [Page 8]

RFC 4252 SSH Authentication Protocol January 2006
Private keys are often stored in an encrypted form at the client
host, and the user must supply a passphrase before the signature can
be generated. Even if they are not, the signing operation involves
some expensive computation. To avoid unnecessary processing and user
interaction, the following message is provided for querying whether
authentication using the "publickey" method would be acceptable.
byte SSH_MSG_USERAUTH_REQUEST
string user name in ISO-10646 UTF-8 encoding [RFC3629]
string service name in US-ASCII
string "publickey"
boolean FALSE
string public key algorithm name
string public key blob
Public key algorithms are defined in the transport layer
specification [SSH-TRANS]. The 'public key blob' may contain
certificates.
Any public key algorithm may be offered for use in authentication.
In particular, the list is not constrained by what was negotiated
during key exchange. If the server does not support some algorithm,
it MUST simply reject the request.
The server MUST respond to this message with either
SSH_MSG_USERAUTH_FAILURE or with the following:
byte SSH_MSG_USERAUTH_PK_OK
string public key algorithm name from the request
string public key blob from the request
To perform actual authentication, the client MAY then send a
signature generated using the private key. The client MAY send the
signature directly without first verifying whether the key is
acceptable. The signature is sent using the following packet:
byte SSH_MSG_USERAUTH_REQUEST
string user name
string service name
string "publickey"
boolean TRUE
string public key algorithm name
string public key to be used for authentication
string signature
Ylonen & Lonvick Standards Track [Page 9]

RFC 4252 SSH Authentication Protocol January 2006
The value of 'signature' is a signature by the corresponding private
key over the following data, in the following order:
string session identifier
byte SSH_MSG_USERAUTH_REQUEST
string user name
string service name
string "publickey"
boolean TRUE
string public key algorithm name
string public key to be used for authentication
When the server receives this message, it MUST check whether the
supplied key is acceptable for authentication, and if so, it MUST
check whether the signature is correct.
If both checks succeed, this method is successful. Note that the
server may require additional authentications. The server MUST
respond with SSH_MSG_USERAUTH_SUCCESS (if no more authentications are
needed), or SSH_MSG_USERAUTH_FAILURE (if the request failed, or more
authentications are needed).
The following method-specific message numbers are used by the
"publickey" authentication method.
SSH_MSG_USERAUTH_PK_OK 60
8. Password Authentication Method: "password"
Password authentication uses the following packets. Note that a
server MAY request that a user change the password. All
implementations SHOULD support password authentication.
byte SSH_MSG_USERAUTH_REQUEST
string user name
string service name
string "password"
boolean FALSE
string plaintext password in ISO-10646 UTF-8 encoding [RFC3629]
Note that the 'plaintext password' value is encoded in ISO-10646
UTF-8. It is up to the server how to interpret the password and
validate it against the password database. However, if the client
reads the password in some other encoding (e.g., ISO 8859-1 - ISO
Latin1), it MUST convert the password to ISO-10646 UTF-8 before
transmitting, and the server MUST convert the password to the
encoding used on that system for passwords.
Ylonen & Lonvick Standards Track [Page 10]

RFC 4252 SSH Authentication Protocol January 2006
From an internationalization standpoint, it is desired that if a user
enters their password, the authentication process will work
regardless of what OS and client software the user is using. Doing
so requires normalization. Systems supporting non-ASCII passwords
SHOULD always normalize passwords and user names whenever they are
added to the database, or compared (with or without hashing) to
existing entries in the database. SSH implementations that both
store the passwords and compare them SHOULD use [RFC4013] for
normalization.
Note that even though the cleartext password is transmitted in the
packet, the entire packet is encrypted by the transport layer. Both
the server and the client should check whether the underlying
transport layer provides confidentiality (i.e., if encryption is
being used). If no confidentiality is provided ("none" cipher),
password authentication SHOULD be disabled. If there is no
confidentiality or no MAC, password change SHOULD be disabled.
Normally, the server responds to this message with success or
failure. However, if the password has expired, the server SHOULD
indicate this by responding with SSH_MSG_USERAUTH_PASSWD_CHANGEREQ.
In any case, the server MUST NOT allow an expired password to be used
for authentication.
byte SSH_MSG_USERAUTH_PASSWD_CHANGEREQ
string prompt in ISO-10646 UTF-8 encoding [RFC3629]
string language tag [RFC3066]
In this case, the client MAY continue with a different authentication
method, or request a new password from the user and retry password
authentication using the following message. The client MAY also send
this message instead of the normal password authentication request
without the server asking for it.
byte SSH_MSG_USERAUTH_REQUEST
string user name
string service name
string "password"
boolean TRUE
string plaintext old password in ISO-10646 UTF-8 encoding
[RFC3629]
string plaintext new password in ISO-10646 UTF-8 encoding
[RFC3629]
Ylonen & Lonvick Standards Track [Page 11]

RFC 4252 SSH Authentication Protocol January 2006
The server must reply to each request message with
SSH_MSG_USERAUTH_SUCCESS, SSH_MSG_USERAUTH_FAILURE, or another
SSH_MSG_USERAUTH_PASSWD_CHANGEREQ. The meaning of these is as
follows:
SSH_MSG_USERAUTH_SUCCESS - The password has been changed, and
authentication has been successfully completed.
SSH_MSG_USERAUTH_FAILURE with partial success - The password has
been changed, but more authentications are needed.
SSH_MSG_USERAUTH_FAILURE without partial success - The password
has not been changed. Either password changing was not supported,
or the old password was bad. Note that if the server has already
sent SSH_MSG_USERAUTH_PASSWD_CHANGEREQ, we know that it supports
changing the password.
SSH_MSG_USERAUTH_CHANGEREQ - The password was not changed because
the new password was not acceptable (e.g., too easy to guess).
The following method-specific message numbers are used by the
password authentication method.
SSH_MSG_USERAUTH_PASSWD_CHANGEREQ 60
9. Host-Based Authentication: "hostbased"
Some sites wish to allow authentication based on the host that the
user is coming from and the user name on the remote host. While this
form of authentication is not suitable for high-security sites, it
can be very convenient in many environments. This form of
authentication is OPTIONAL. When used, special care SHOULD be taken
to prevent a regular user from obtaining the private host key.
The client requests this form of authentication by sending the
following message. It is similar to the UNIX "rhosts" and
"hosts.equiv" styles of authentication, except that the identity of
the client host is checked more rigorously.
This method works by having the client send a signature created with
the private key of the client host, which the server checks with that
host's public key. Once the client host's identity is established,
authorization (but no further authentication) is performed based on
the user names on the server and the client, and the client host
name.
Ylonen & Lonvick Standards Track [Page 12]

RFC 4252 SSH Authentication Protocol January 2006
byte SSH_MSG_USERAUTH_REQUEST
string user name
string service name
string "hostbased"
string public key algorithm for host key
string public host key and certificates for client host
string client host name expressed as the FQDN in US-ASCII
string user name on the client host in ISO-10646 UTF-8 encoding
[RFC3629]
string signature
Public key algorithm names for use in 'public key algorithm for host
key' are defined in the transport layer specification [SSH-TRANS].
The 'public host key and certificates for client host' may include
certificates.
The value of 'signature' is a signature with the private host key of
the following data, in this order:
string session identifier
byte SSH_MSG_USERAUTH_REQUEST
string user name
string service name
string "hostbased"
string public key algorithm for host key
string public host key and certificates for client host
string client host name expressed as the FQDN in US-ASCII
string user name on the client host in ISO-10646 UTF-8 encoding
[RFC3629]
The server MUST verify that the host key actually belongs to the
client host named in the message, that the given user on that host is
allowed to log in, and that the 'signature' value is a valid
signature on the appropriate value by the given host key. The server
MAY ignore the client 'user name', if it wants to authenticate only
the client host.
Whenever possible, it is RECOMMENDED that the server perform
additional checks to verify that the network address obtained from
the (untrusted) network matches the given client host name. This
makes exploiting compromised host keys more difficult. Note that
this may require special handling for connections coming through a
firewall.
Ylonen & Lonvick Standards Track [Page 13]

RFC 4252 SSH Authentication Protocol January 200610. IANA Considerations
This document is part of a set. The IANA considerations for the SSH
protocol, as defined in [SSH-ARCH], [SSH-TRANS], [SSH-CONNECT], and
this document, are detailed in [SSH-NUMBERS].
11. Security Considerations
The purpose of this protocol is to perform client user
authentication. It assumed that this runs over a secure transport
layer protocol, which has already authenticated the server machine,
established an encrypted communications channel, and computed a
unique session identifier for this session. The transport layer
provides forward secrecy for password authentication and other
methods that rely on secret data.
Full security considerations for this protocol are provided in
[SSH-ARCH].
Ylonen & Lonvick Standards Track [Page 14]

RFC 4252 SSH Authentication Protocol January 2006
Full Copyright Statement
Copyright (C) The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Acknowledgement
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
Ylonen & Lonvick Standards Track [Page 17]